Ventilation systems of this type are frequently used in the field of rotary electrical machines, in particular in alternators for motor vehicles comprising a casing in at least two parts, referred to as the front bearing and rear bearing. These bearings are provided with an inlet and outlet openings for the circulation of a cooling fluid inside the machine under the action of at least one fan.
More precisely, this fan, fixed to the rotor, creates a flow of cooling fluid, such as air, the flow rate of which increases linearly with the speed of rotation. This linear change in the air flow with the speed of rotation gives rise to very high outputs at high speeds of rotation, per minute, providing effective cooling, in particular of the stator, at these speeds.
However, these so high flow rates have the counterpart that the power P consumed is very great, for example around 775 watts for a flow rate of around 80 litres per second at 18000 revolutions per minute, for a given rear fan. Thus, for a rotary electrical machine equipped with a front fan and a rear fan, in the present case for example, there therefore occurs a total mechanical loss of around 1.5 kW. Another major drawback of the conventional ventilation systems described above is that the ventilation noise level, which appears at around 7000 revolutions per minute (rpm), becomes very high, for example 115 dB at a speed of rotation of 18000 (rpm). FIG. 1 makes it possible to asses, for a given conventional rotary electrical machine, the change in the useful power indicated by curve A and the consumed power indicated by curve B, as a function of the speed of rotation. This figure shows clearly the unsatisfactory ratio of the useful power in comparison with the consumed power, inherent in the conventional ventilation system using a fan rotationally fixed to the rotor.